Method of manufacturing polyoxymethylene filaments

ABSTRACT

A method of manufacturing polymethylene filaments, wherein polyoxymethylene having the molecular weight from 30,000 to 100,000 and containing stabilizing additives in a quantity from 0.1 to 3.0 per cent of the weight of the polyoxymethylene is subjected to thermal treatment at a temperature from 100° to 150° C. and residual pressure from 1 to 100 mm mercury to attain constant weight. The thus thermally treated polyoxymethylene is melted at a temperature from 170° to 230° C., whereafter the melt is forced through the orifices of an extrusion nozzle. The jets of the melt, leaving the orifices of the extrusion nozzle, are cooled at a temperature from 70° to 169° C. After the cooling the obtained filaments are drawn at a temperature from 120° to 165° C. to a length exceeding from 7 to 14 times the initial length. 
     The disclosed method offers a simple technology of producing filaments as strong as 100 grams per tex. 
     To produce low-shrinkage fiber (i.e. with a shrinkage rate from 0 to 5 per cent at 150° C.) the drawn filaments are thermally treated at a temperature exceeding that of the drawing by 2° C. to 50° C., the filaments being maintained under tension. Alternatively, the drawn filaments may be first tensioned and then thermally treated, as indicated hereinabove.

A method of manufacturing polymethylene filaments, whereinpolyoxymethylene having the molecular weight from 30,000 to 100,000 andcontaining stabilizing additives in a quantity from 0.1 to 3.0 percentof the weight of the polyoxymethylene is subjected to thermal treatmentat a temperature from 100° C to 150° C and residual pressure from 1 to100 mm mercury to attain constant weight. The thus thermally treatedpolyoxymethylene is melted at a temperature from 170° C to 230° C.,whereafter the melt is forced through the orifices of an extrusionnozzle. The jets of the melt, leaving the orifices of the extrusionnozzle, are cooled at a temperature from 70° C. to 169° C. After thecooling the obtained filaments are drawn at a temperature from 120° C to165° C. to a length exceeding from 7 to 14 times the initial length.

The disclosed method offers a simple technology of producing filamentsas strong as 100 grams per tex.

To produce low-shrinkage fibre (i.e. with a shrinkage rate from 0 to 5percent at 150° C.) the drawn filaments are thermally treated at atemperature exceeding that of the drawing by 2° to 50° C., the filamentsbeing maintained under tension. Alternatively, the drawn filaments maybe first tensioned and then thermally treated, as indicated hereinabove.

The present invention relates to the methods of manufacturingpolyoxymethylene filaments, and, more particularly, of high-strengthlow-shrinkage polyoxymethylene filaments.

These filaments can be widely used in the production of fishing nets andtrawls, of filtering cloth, of engineering rubber articles, cord, etc.,since they offer a whole series of valuable properties. Thus, amongtheir properties is the one of being hydrophobic or water-repellent,which means that their strength is unaffected by moisture; they areproof to the action of alkali, as well as of numerous organic solventsat a temperature up to 100° C.; they are likewise proof to the action ofsea water and are biologically stable. Polyoxymethylene filaments alsooffer high tensile strength, resistance to rubbing, fatigue strength andelasticity.

Furthermore, polyoxymethylene filaments and yarn can be widely used inthe production of numerous textile articles, e.g. in the form oftexturized or bulk yarn.

There already exists a number of methods of manufacturingpolyoxymethylene filaments from melted polyoxymethylene by moulding afilament with subsequent drawing.

The high viscosity of the melt and the high crystallization rate ofpolyoxymethylene are reflected in the specific features of itsprocessing into filaments. The relatively low temperature viscosityfactor and the relatively low thermal stability of the melt would notpermit to reduce the viscosity of the melt any considerably byincreasing the temperature. Particular difficulties are encountered atprocessing of polyoxymethylene with a high molecular weight, which isthe one generally used for manufacturing polyoxymethylene filaments withhigh physical and mechanical properties, such as elasticity, fatiguestrength, etc. It is this high viscosity of the melt ofpolyoxymethylene, particularly, of polyoxymethylene with a highmolecular weight, which dooms the rate of extrusion of the filaments tobe substantially lower than that of extrusion of filaments of othermaterials, usually attained in the art of making man-made fibre.

There is known a method of manufacturing polyoxymethylene filaments,wherein, in order to step up the extruding speed (and, consequently, thewinding speed) there is effected "cooling" of the jets of the melt,leaving the orifices of the extrusion nozzle, at a temperature from 170°to 240° C. (see Japan Pat. No. 3486, Cl. 42D22). From the describedexamples of this method it can be seen that raising the temperature ofthe air in the vicinity of the extrusion nozzle from normal to 190° C.enables to step up the extruding speed from 160 m/min to 670 m/min.However, the ratio of subsequent drawing of the filament thus obtainedat a temperature of 150° C. does not exceed 7:1, and, consequently, thestrength of this filament is but 67.5 grams per tex with e longation atrupture about 20 percent.

There is known another method of manufacturing polyoxymethylenefilaments from melted stabilized homo- and co-polymers of formaldehydeor else of its cyclic trimer - trioxane - with cyclic esters, e.g.ethylene oxide, 1,3 dioxolane, etc. (see British Pat. No. 995,848, Cl. D01 f, D 06 m, C 08 g). Stabilizing additives are included in a quantity,for example, of 0.1 to 3.0 percent of the weight of the polymer. Toreduce the viscosity of the melt there is sometimes added into thestabilized polymer a certain quantity of a plastifier. To mouldpolyoxymethylene filaments, the said homo- or co-polymers are subjectedto melting at a temperature from 170° C. to 230° C. and at the sametemperature the melted polymer is forced through the orifices of anextrusion nozzle. The jets of the melt, leaving the orifice, are cooledin the ambient air. After the cooling, the moulded filament is subjectedto drawing. By drawing the moulded filament at a rate of 10.2 m/min andtemperature from 120° to 150° C., e.g. 134° C., the draft, for example,being 9.05 : 1, there is obtained a filament with the strength not inexcess of 54.9 grams per tex. In order to step up the strength of thefilament, it is subjected to a repeated drawing at a rate of 10.5 m/minat a temperature from 150° to 160° C., the draft being from 105 : 1 to2:1. As a result, the strength is increased to 89.1 grams per tex.

The use of plastifiers in certain cases involves the necessity ofresorting to additional labour-consuming operations of removing theplastifier. On the other hand, the presence of the plastifier in a finalfilament considerably affects its physical and mechanical properties.

To obtain filaments of a sufficiently high strength by thelast-described method, the moulded filament is drawn not in a singlestage, but in two stages, which also complicates the technology.Besides, this drawing is effected at a relatively low speed, whichlowers the productivity of the equipment.

Furthermore, the cooling of the jets leaving the nozzle in the ambientair would not permit to mould the filaments at high speeds, whichbecomes particularly pronounced, when polyoxymethylene of a highmolecular weight is processed.

It is an object of the present invention to develop a method ofmanufacturing polyoxymethylene filaments, which should provide forobtaining polyoxymethylene filaments with high physical and mechanicalproperties.

It is another object of the present invention to simplify the technologyof the manufacturing process.

With these and other objects in view, the present invention resides in amethod of manufacturing polyoxymethylene filaments, including melting ata temperature from 170° to 230° C, a mass of polyoxymethylene having themolecular weight from 30,000 to 100,000 and containing stabilizingadditives in a quantity of 0.1 to 3.0 percent of the weight of thepolyoxymethylene mass, forcing the melt thus obtained through theorifices of an extrusion nozzle, cooling the jets of the melt, leavingthe extrusion nozzle and drawing the thus moulded filaments at atemperature from 120° to 165° C. to a length exceeding the initiallength from 7 to 14 times. In accordance with the invention, the saidmass of polyoxymethylene, containing the said additives, is subjectedprior to the melting to a thermal treatment at a temperature from 100°to 150° C. and residual pressure from 1 to 100 mm mercury to a constantweight, and the jets of the melt, leaving the orifices of the extrusionnozzle, are cooled at a temperature from 70° to 169° C.

For the filament-forming polyoxymethylene there are used either homo- orco-polymers of formaldehyde or else of its cyclic trimer - trioxane -with cyclic esters of a general formula, for example ##STR1## where "n"is an integer within a range from 0 to 2. The weight content of thesecond co-monomer in the co-polymer may vary within a range from 0.5 to10 percent, depending on the destination of the filaments to bemanufactured.

The molecular weight (M_(w)) of the polyoxymethylene used may vary from30,000 to 100,000 and is calculated from the formula: [τ]=K.M_(w).sup.αwhere [τ] is the characteristic viscosity of solution ofpolyoxymethylene in dimethylformamide, measured at 150° C. ± 0.5° C onOstwald-Pinkevich viscosimeter; K equals 4.4 . 10⁻⁴ and α equals 0.66.

It is not advisable to use polyoxymethylene with a molecular weightbelow 30,000, since the strength of filaments produced from such apolymer is insufficient. On the other hand, when polyoxymethylene with amolecular weight in excess of 100,000 is used, there are encounteredcertain technological difficulties on account of the high viscosity ofthe melt.

An increased content of the second co-monomer in the copolymer leads toa lower melting point of the co-polymer, and, consequently, to a reducedthermal strength of the filament.

To step up the thermal stability of polyoxymethylene, there areintroduced thereinto stabilizing additives in a quantity from 0.1 to 3.0percent of the weight of the mass of polyoxymethylene. For thestabilizers can be used, for instance, a system of two componentsincluding an antioxidant and a acceptor of formaldehyde. Among theantioxidants are such substances as bis-phenols, e.g. 2,2'-methylene bis(4-methyl-6-tertiary butyl-phenol); 4,4'-butyledene bis-(6-tertiarybutyl-4-methyl-phenol), while among the acceptors are polyamides,polyurethanes, compositions containing tertiary amines and final amidegroups, for example, dicyandiamide (cyanguanidine).

The properties of the filament are to a great extent dependent on theinitial raw materials and the technological conditions of theirmanufacture.

Stabilized polyoxymethylene may contain up to 0.2 percent by weight ofequilibrium moisture (water). The presence of the moisture (water) inthe polymer adversely affects the moulding and drawing operations andthe quality of the filaments. An increased content of water leads toformation of bubbles of steam in the jets of the melt, leaving theorifices of the extrusion nozzle, which might result in breakage of thejets in the moulding operation, or else in breakage of the filamentduring subsequent drawing.

Under the herein disclosed conditions of thermal treatment of theinitial mass of polyoxymethylene to a constant weight, i.e. at atemperature from 100° to 150° C. and residual pressure from 1 to 100 mmmercury, it has been quite unexpectedly found that the loss of theweight by the polymer substantially exceeds the content therein of theequilibrium water. It has been also found that the strength of thefilament produced from the polymer subjected to this thermal treatmentis enhanced. The loss of the weight by the polymer at this thermaltreatment may be explained by the fact that in addition to water thereare removed from the mass of polyoxymethylene several othercompositions: on the one hand, there is formaldehyde mechanically addedto the polyoxymethylene during granulation of the latter; on the otherhand, there is formaldehyde breaking from the unstable portion of themacromolecules of polyoxymethylene. Besides, it is not altogetherimprobable that under the above conditions there are partially removedthe stabilizing additives which had been introduced earlier.

The presence of volatile compositions in the mass of polyoxymethylenelikewise adversely affects the processing properties of the polymer, asit is the case with water. The presence of volatile compositions in themass of polyoxymethylene, on the one hand, influences the permissiblerange of melting temperatures. With a high content of volatilesubstances there might appear in the melt an amount of bubbles which isthe greater, the higher is this melting temperature. Consequently, witha reduced content of volatile substances in the mass ofpolyoxymethylene, which is attained by the abovedescribed preliminarythermal treatment, it is fairly permissible to raise the meltingtemperature. On the other hand, the smaller the content of volatilesubstances in the melt of polyoxymethylene, the smaller is the degree oftheir evolution from the jets of the melt, leaving the orifices of theextrusion nozzle, and, consequently, the smaller is the probability ofjet breakage.

The Table hereinbelow contains data on the amount of volatile substancesremoved from the stabilized co-polymer of trioxane and 1,3 dioxolane(the latter contained in a quantity of 4 percent by weight), as well ason the amount of water left in the co-polymer, depending on the thermaltreatment conditions.

    ______________________________________                                                     Amount                                                                        of vol-                                                                       atile                                                                         subst-                                                                        ances     Residual  Amount -Residual removed content of of                                        volatile                                     content      from      water in  substances                                   of water     specimen, specimen  removed                                      in specimen, % by      % by      from specimen                                % by weight  weight    weight    % by weight                                                         Thermal treatment in                                         Thermal treatment in                                                                           vacuum, with                                           Temp. air under normal residual pressure from                                 ° C                                                                          pressure         1 to 5 mm mercury                                      ______________________________________                                         20   0.12       --        0.12    --                                         100   0.04       0.16      traces  0.26                                       140   0.02       0.42      d.t.o.  0.62                                       155   traces     0.55      d.t.o.  0.70                                       ______________________________________                                    

It can be seen from the above data that the amount of removed volatilesubstances grows with the increase of the temperature of the treatmentand with the reduction of the pressure, whereas the water content ispractically unaffected by these conditions. The water content wasdetermined by a method based on reduction of iodine in the presence ofwater by sulphur dioxide, known as the Fischer method (see "Control overproduction of chemical fibre", "Khimiya" Publishers, Moscow, 1967, p.293). The amount of the removed volatile substances was determined asthe difference between the weight of the specimen prior to and after thethermal treatment.

Moulding of the filaments is effected by melting the stabilizedpolyoxymethylene which had been subjected to the thermal treatment,forcing the melt through the orifices of an extrusion nozzle and coolingdown the jets of the melt, leaving the orifices of the nozzle. Themelting temperature can be in the range from 170° to 230° C., dependingon the nature of the polyoxymethylene being processed, on its molecularweight and the time of its existence in the melted state, determined bythe capacity of the equipment used. The cooling down of the jets of themelt in the vicinity of the nozzle is effected at a temperature from 70°to 169° C. Under the herein disclosed "mild" cooling conditions, thegeneral molecular orientation of the composition is affected and,consequently, the drawing ability of the moulded filaments is stepped up(the draft can be increased), as compared with the drawing ability offilaments moulded at a lower cooling temperature, e.g. at 20° C. Thephenomenon of reduction of general molecular orientation at processingof polyoxymethylene of a high molecular weight, as high as 50,000 to100,000, is particularly pronounced. Filaments moulded at higher coolingtemperatures and subjected to greater draft offer better physical andmechanical properties.

The cooling media can be air, an inert gas, e.g. nitrogen, steam.

After the cooling the moulded filament is drawn at a temperature from120° to 165° C. to a length exceeding the initial length 7 to 14 times.At temperatures below 120° C. it is not possible to drawpolyoxymethylene to a degree providing for production of filaments withadequately high strength. This fact is related to the high degree ofcrystallinity of polyoxymethylene, as well as to the large size of theabovemolecular structure. To break up this structure for drawingpurposes, it is essential that the filament should be heated up to atemperature not below 120° C. As the temperature is raised above 145° -150° C., there takes place maximally complete breaking up of the initialabove-molecular structure, which simplifies the task of itsre-arrangement and re-orientation, and, consequently, the attainablevalue of draft is sharply increased. Under these conditions it becomespossible to draw the moulded filament to a length exceeding the initiallength from 9 to 14 times even by single-stage drawing, which providesfor high physical and mechanical properties of the filament.

As the drawn polyoxymethylene filaments are worked into various articlesand during operation of these articles under the action of elevatedtemperatures, there might be developed within these filamentsconsiderable internal stresses, as high as 10 kg/mm². Thus, if thefilament during its thermal treatment is not tensioned, these internalstresses might lead to shrinkage of the filament, and at the same timethere might take place reduction of the tensile strength and increasingof the value of elongation at rupture.

In order to reduce shrinkage of the produced filaments at subsequentthermal treatment, as well as to maintain the strength of the filamentsafter such subsequent thermal treatment it is advisable that the drawnpolyoxymethylene filaments should be subjected to a thermal treatment ata temperature 2° to 50° C. above the temperature at which the filamentswere drawn, with the filaments being in a tensioned state. It ispossible first to tension the drawn filaments and then to subject themto the said thermal treatment.

As it has been already stated hereinabove, in the process of thermaltreatment of a filament manufactured from thermally pre-treatedstabilized polyoxymethylene under the abovespecified conditioned noreduction of the original strength of the filament is encountered.However, in the process of similar thermal treatment of a filamentmanufactured from the same stabilized polyoxymethylene pre-treated underdifferent conditions, e.g. in open air at a temperature of 140° C., thefilament has been found to lose its strength after the abovespecifiedthermal treatment thereof.

The herein disclosed method is simple technologically. It does notrequire any specific equipment. The method provides for processing intofilaments polyoxymethylene of a high molecular weight without the use ofplastifiers (i.e. substances reducing the viscosity of the melt ofpolyoxymethylene) the addition of which brings about the necessity ofcarrying out an additional operation connected with removal of theseplastifiers from the filament. Furthermore, a filament produced fromthermally pre-treated polyoxymethylene and moulded under the raidcooling conditions is characterized by low general molecular orientationand a minimal content of gas-like impurities. Therefore, it can be drawnunder a given temperature duty even in a single stage, with high draftand high drawing speeds (as high as 100 m/min) which further simplifiesthe technology and increases the productivity of the equipment.

The herein disclosed method provides for manufacturing polyoxymethylenefilaments having high physical and mechanical properties. The tensilestrength of the filaments is within a range from 70 to 100 grams pertex, with elongation at rupture from 9 to 12 percent, the initialmodulus being from 1200 to 1800 kilograms per square millimeter. Thefilaments offer an increased thermal stability and stability to theaction of acids, high fatigue strength and rubbing resistance. Moreover,the herein disclosed method provides for reducing considerably thedegree of shrinkage of a drawn filament, due to the filament having beensubjected to tensioning and thermal treatment.

The method of manufacturing polyoxymethylene filaments is carried out,as follows.

An initial mass of polyoxymethylene, e.g. in the form of granules,containing stabilizing additives, is thermally pre-treated on a standardequipment, e.g. in vacuum drum-type dryers at a temperature from 100° to150° C and residual pressure from 1 to 100 mm mercury to constantweight. The said thermal treatment may be effected in an inert gasatmosphere, e.g. in a nitrogen atmosphere. In this case the temperatureof thermal pre-treatment of polyoxymethylene may be stepped up to 160°to 165° C., and the operation may be carried out under atmosphericpressure.

Thereafter the thermally pre-treated polyoxymethylene is melted,preferably, in an extrusion machine. The melting is performed at atemperature from 170° to 230° C., depending on the chemical nature andmolecular weight of the polyoxymethylene used. The melting operation maybe carried out either in air or in an inert gas atmosphere, e.g. in anitrogen atmosphere. The temperature of melting depends on the nature ofthe polyoxymethylene used, on the atmosphere in which the melting isperformed, on the design on the equipment and may vary from 0.1 to 60minutes.

The thus obtained polyoxymethylene melt is fed by a metering pump to anextrusion nozzle having either one or several orifices. Prior to beingsupplied to the nozzle, wherever necessary, the melt may be filteredthrough metal filtering screens, quartz sand or any other suitablefiltering means.

The jets of the melt, leaving the orifices of the extrusion nozzle, arecooled, e.g. in an atmosphere of air heated to 70° - 169° C. This may beperformed in a closed heated shaft, or else by directing a steam ofheated gas, as it is being generally done at the manufacturing of amajority of known synthetic filaments produced by moulding from apolymer melt.

The filament leaving the cooling zone has applied thereon a lubricant,water or any other substance, depending on the destination of thefilament.

Then the filament is either wound into a package or else this stage isby-passed, and the filament is fed directly into a drawing machine. Inother words, the herein disclosed method may be performed both in anintermittent and continuous mode.

Drawing or drafting of the filament is effected in a single stage at atemperature from 120° to 165° C. to a 7:1 to 14:1 draft. The drawingoperation is performed by a commonly known system including a feedroller and a drafting one, the circumferential speed of the draftingroller being higher than that of the feeding one. Heating of thefilament is effected intermediate of the rollers. The filament can beheated by contact heaters of the "iron" type, or else by passing througha heated tube (with either hot air or radiation heating); alternatively,the filament may pass through a heated liquid. The drawn filament,depending on its destination, may be either twisted or not twisted. Thetwisting may be performd by any suitable known twisting mechanism, e.g.of the ring twisting kind. It is also possible to cut the drawn filamentinto stable fibre of a required length.

To obtain low-shrinkage polyoxymethylene filament, the drawn filament istensioned and thermally treated at a temperature 2° to 50° C. above thedrawing temperature. The thermal treatment may be performed in acontinuous "drawing-cum-thermal treatment" process. In this case thermaltreatment of the filament may be effected by various existing systemsproviding for tensioning the filament and heating it to requiredparameters, e.g. a system including a pair of rollers intermediate ofwhich there is ensured a required tension and a heater is provided. In acase of an intermittent process any suitable technique may be used, e.g.thermal treatment of the filament wound at a required tension on a rigidbobbin. The thermal treatment of the filament may be performed invarious atmospheres (gaseous or liquid) wherein no chemical destructionof polyoxymethylene takes place, or else in air. The time and parametersof the thermal treatment of the filament are defined by the requirementsas to the value of permissible shrinkage of final filaments.

For the present invention to be better understood, there are describedhereinbelow several examples.

EXAMPLE 1

Granulated co-polymer of trioxane and 1,3-dioxolane (the weight contentof the latter being 4 percent) with additives: 2,2'-methylene bis(4-methyl-6-tretbutylphenol) in a quantity of 0.5 percent by weight asthe antioxidant and formaldehydedicyandiamide in a quantity of 0.5percent by weight as the acceptor, with characteristic viscosity indimethylformamide equalling 0.56 is thermally treated at 100° C. andresidual pressure of 5 mm mercury, melted in an extrusion machine in anair atmosphere at 190° C., and the melt is forced by a metering pumpthrough an extrusion nozzle with a single orifice 1.2 mm in diameter.The rate of feed of the melt is 5 gr/min. The jet of the melt is cooledin a 0.5 m long tube. The temperature of the air cooling the jet of themelt is 90° C. The filament moulding rate is 250 m/min.

The moulded filament is drawn to a 10:1 draft at a speed of 125 m/minover a 250 mm long "iron" at 150° C. The obtained filament has 72 gramsper tex tensile strength and elongation at rupture equalling 11.5%. Theshrinkage of the filament is 18%, with 80% of the initial strengthremaining after the shrinkage.

After the drawing operation the filament is thermally treated in airunder a tension of 2 kilograms per square millimeter at 155° C. for 30minutes. After this treatment the shrinkage value is 4%, with 95% of theinitial strength remaining after the shrinkage.

EXAMPLE 2

Polyoxymethylene filaments are moulded and drawn, as described above inExample 1, with a difference that the initial mass of polyoxymethyleneis thermally treated at 130° C. and residual pressure of 10 mm mercury;the co-polymer is melted at 200° C., and the jet of the melt is cooledin air at a temperature of 70° C.

The filament obtained features 75 gr/tex tensile strength and 11.1%elongation at rupture. The shrinkage rate of this filament is 17%, with82% of the initial strength remaining after the shrinkage. After thedrawing operation the filament is thermally treated in a nitrogenatmosphere at 161° C. under 6 kg/mm² tension for 20 minutes. After thistreatment the shrinkage rate of the filament is 1%, with 96% of theinitial strength remaining after the shrinkage.

EXAMPLE 3

Polyoxymethylene filaments are moulded and drawn, as described above inExample 1, with a difference that a co-polymer of formaldehyde and1,3-dioxolane is used, having characteristic viscosity indimethylformamide equalling 0.65; the co-polymer is thermally treated at150° C. and 1 mm mercury residual pressure; the co-polymer is melted at185° C., and the jet of the melt is cooled in air at 140° C. in a 0.4 mlong tube. The moulded filament is drawn to a 12:1 draft at a speed of75 m/min over a 400 mm long "irong".

The filament obtained has 86 gr/tex tensile strength and 10% elongationat rupture. The shrinkage rate of this filament is 15%, with 87% of theinitial strength remaining after the shrinkage.

After the drawing the filament is thermally treated at 170° C. in anitrogen atmosphere under 10 kg/mm² tension for 5 minutes. Followingthis treatment the shrinkage is 0.5% with 95% of the initial strengthremaining after the shrinkage.

EXAMPLE 4

A polyoxymethylene filament is moulded as described above in Example 3,a difference being in that the co-polymer is thermally treated at 140°C. and residual pressure of 95 mm mercury; the co-polymer is melted at180° C; the jet of the melt is cooled in air at 120° C.

The moulded filament is drawn to a 13.6:1 draft at a speed of 25 m/minover a 1000 mm long "iron" at 158° C.

The filament obtained is characterized by 98 gr/tex tensile strength and9% elongation at rupture. The shrinkage rate of the filament is 11%,with 90% of the initial strength remaining after the shrinkage.

After the drawing the filament is thermally treated at 150° C under 0.2kg/mm² tension for 150 minutes. Following the treatment, the shrinkageof the filament is 3%, with 94% of the initial strength remaining.

EXAMPLE 5

A polyoxymethylene filament is moulded and drawn, as described above inExample 1, with a difference that there is used a copolymer of trioxaneand a cyclic ester, i.e. ethylene oxide (the weight content of thelatter being 3 percent), having characteristic viscosity indimethylformamide equalling 0.50; the co-polymer is thermally treated at130° C. and residual pressure equalling 50 mm mercury; the co-polymer ismelted at 170° C., and the melt is forced through an extrusion nozzlehaving 24 orifices 0.40 mm in diameter. The rate of feed of the melt is20 grams per minute. The jets of the melt are cooled in a 0.7m long tubewith air at 100° C. The filament moulding rate is 450 m/min.

The filament obtained is characterized by 76 gr/tex tensile strength and14% elongation at rupture. The shrinkage of this filament is 20%, with75% of the initial strength remaining after the shrinkage.

After the drawing the filament is thermally treated in a nitrogenatmosphere at 152° C. under 4 kg/mm² tension for 240 minutes.

Following this treatment the shrinkage is 5%, with 92% of the initialstrength remaining.

EXAMPLE 6

Polyoxymethylene filaments are moulded and drawn, as described above inExample 5, with a difference that the temperature of the air cooling thejets of the melt is 70° C.

The moulded filament is drawn to a 7 : 1 draft at a 10 m/min speed overa 700 mm long "iron" at 120° C.

The filament obtained has 62 gr/tex tensile strength, with 16%elongation at rupture. The shrinkage rate of this filament is 25%, with67% of the initial strength remaining after the shrinkage.

After the drawing the filament is thermally treated in a nitrogenatmosphere at 150° C. under 8 kg/mm² tension.

Following this treatment the shrinkage rate at 150° C. is 10%, with 85%of the initial strength remaining.

EXAMPLE 7

A co-polymer of formaldehyde and 1,3-dioxolane (the weight content ofthe latter being 5%) in a powder form is mixed with stabilizingadditives, viz. 2,2'-methylene bis (4 methyl-6-tertbutylphenol) in aquantity of 2.5% by weight as the antioxidant andformaldehyde-dicyanamide in a quantity of 0.5% by weight as theacceptor; the co-polymer is thermally treated, as described above inExample 5 and melted at 170° C. The co-polymer used has characteristicviscosity of 0.42.

The filament is moulded, as described hereinabove in Example 6, anddrawn to a 8:1 draft in a contactless 800 mm long heater in anatmosphere of air heated to 130° C. at a speed of 50 m/min.

The filament obtained is characterized by 59 gr/tex tensile strength and20% elongation at rupture.

EXAMPLE 8

A granulated homo-polymer of formaldehyde, containing 0.5% by weightco-polymer based on hexamethylenediamine, adipinic acid and caprolactamand 0.5% by weight dicyanamide, having characteristic viscosity indimethylformamide equalling 0.86, is thermally treated at 110° C. and0.5 mm mercury pressure. Then the polymer is melted in a nitrogenatmosphere at 210° C., and the melt thus obtained is forced through anextrusion nozzle with 12 orifices 0.8 mm in diameter, the rate of feedof the melt being 20 grams a minute. The jets of the melt are cooledwith nitrogen heated to 165° C. The filament moulding speed is 500m/min.

The moulded filament is drawn to a 8:1 draft at a rate of 15 m/min overa 700 mm long "iron" at 161° C.

The filament obtained has 71 gr/tex tensile strength and elongation atrupture equalling 12%.

EXAMPLE 9

A polyoxymethylene filament is moulded, as described above in Example 1,with a difference that the polyoxymethylene is melted in a nitrogenatmosphere at 225° C., and the melt thus obtained is forced through anextrusion nozzle having 80 orifices 0.5 mm in diameter, the rate of feedof the melt being 150 gr/min. The temperature of the air cooling thejets of the melt is 80° C. The filament moulding speed is 300 m/min.

The moulded filament is drawn to a 9:1 draft over a 500 mm long "iron"at 150° C.

The filament obtained in characterized by 75 gr/tex tensile strength and13% elongation at rupture. The shrinkage of this filament is 17%, with82% of the initial strength remaining after the shrinkage.

After the drawing the filament is thermally treated in air at 157° C.under 8 kg/mm² tension for 40 minutes.

Following this treatment the shrinkage rate of the filament is 4%, with92% of the initial strength remaining.

In the above examples the tensile strength was determined on apendulum-type rupturing machine, with filament elongation rate being 500mm/min and clamping length being 250 mm.

The shrinkage rate of the filament is determined from the ratio:

    shrinkage rate equals 1.sub.1 - 1.sub.2 /1.sub.1. 100%, where

1₁ is the initial length of the specimen, equalling 250 mm;

1₂ is the length of the specimen after the filament has been heatedslack to 150° C. in air for 0.5 hours.

The strength of the filament, remaining after the shrinkage, isdetermined from the ratio:

    remaining strength of the filament equals P.sub.2 /P.sub.1. 100%,

where P₁ is the strength of the initial tensioned filament, in grams pertex;

P₂ is the strength of the tensioned filament after the shrinkage, alsoin grams per tex.

What we claim is:
 1. A method of manufacturing a polyoxymethylenefilament, comprising thermally treating polyoxymethylene having amolecular weight of from 30,000 to 100,000 and containing stabilizingadditives including an antioxidant and an acceptor of formaldehyde in aquantity from 0.1 to 3.0% of the weight of said polyoxymethylene at atemperature of from 100° to 150° C and a residual pressure of from 1 to100 mm. Hg to constant weight, melting said thermally treatedpolyoxymethylene at a temperature of from 170° to 230° C, forcing themelt through the orifices of an extrusion nozzle, cooling the jets ofsaid melt leaving the orifices of the extrusion nozzle to a temperatureof from 70° to 169° C in a cooling medium selected from the groupconsisting of air, an inert gas and steam, and drawing the mouldedfilament obtained after said cooling at a temperature of from 120° to165° C to a length exceeding the initial length by about 7 to 14 timesto obtain a polyoxymethylene filament having a tensile strength of from70 to 100 grams per tex, an elongation at rupture of from 9% to 12%, andan initial modulus of from 1200 to 1800 kilograms per square millimeter.2. A method of manufacturing a polyoxymethylene filament comprisingthermally treating polyoxymethylene having a molecular weight of from30,000 to 100,000 and containing stabilizing additives comprising anantioxidant and an acceptor of formaldehyde in a quantity of from 0.1 to3.0% of the weight of said polyoxymethylene at a temperature of from100° to 150° C and at a residual pressure of from 1 to 100 mm. Hg toconstant weight, melting said thermally treated polyoxymethylene at atemperature of from 170° to 230° C, forcing the melt through theorifices of an extrusion nozzle, cooling the jets of said melt leavingthe orifices of the extrusion nozzle to a temperature of from 70° to169° C in a cooling medium selected from the group consisting of air, aninert gas and steam, drawing the moulded filament obtained after saidcooling at a temperature of from 120° to 165° C to a length exceedingthe initial length by about from 7 to 14 times to obtain apolyoxymethylene filament having a tensile strength of from 70 to 100grams per tex, an elongation at rupture of about from 9% to 12%, and aninitial modulus of about from 1200 to 1800 kilograms per squaremillimeter, and thermally treating the drawn filament, while maintainingit under tension, at a temperature of from 2° to 50° C above thetemperature at which the filament was drawn to produce apolyoxymethylene filament having a shrinkage rate of from 0 to 5% at150° C.
 3. The method of claim 2 wherein the jets of said melt arecooled by directing a stream of heated gas past said jets.